The DNA-histone complex (chromatin)

Where the association of histones with DNA is weak, the DNA-histone complex is less condensed (loosely packed). In this condition the DNA is accessible by transcription factors, which can initiate production of mRNA, that is, can switch the gene on. Where this association is stronger, the DNA-histone complex is more condensed (tightly packed). In this condition the DNA is not accessible by transcription factors, which therefore cannot initiate production of mRNA, that is, the gene is switched off. Condensation of the DNA-histone complex therefore inhibits transcription. It can be brought about by decreased acetylation of the histones or by methylation of DNA. Let us turn our attention to these two processes and how they inhibit transcription.